Hi Joekr13,
There's nothing inherently 'wrong' with buying used books, but if the prior user has 'marked up' the book, shown work, circled answers, etc. then YOUR experience with that material might be hindered.
As far as your questions:
1) Re-taking a CAT that you've already used is unrealistic and you'll likely see an 'inflated', unrealistic score the second/third/fourth time around. As such, you should plan NOT to retake a CAT (if you're looking for a realistic score result) - you can certainly retake CATs for the *new* practice problems that you'll see, but the score is almost certain to be inaccurate in those situations.
2) Any used book can potentially have hand-written notes in it, so I'd ask the person who you're buying the books from if the books are 'marked up.'
3) The OG13 and GMAT2015 are the exact same book (with everything in the exact same order), so either one will provide the same content.
When are you planning to take the GMAT?
What is your goal score?
GMAT assassins aren't born, they're made,
Rich